1. Field
The invention is concerned with systems and methods for pulverizing solid fuels, such as coal or other fossil fuels, and for burning such pulverized fuels suspended in a stream of air.
2. State of the Art
The combustion of solid fuels in pulverized form in furnaces has been practiced for many decades, probably beginning with the simple blowing of finely divided coal through pipes directly into the furnace combustion chamber to supplement the normal furnace fire for enhanced temperature and heat generation. Ignition of such supplementary coal came from the heat of the main fire, and little if any consideration was given to the control of fuel flow rates or fuel/air ratios necessary to achieve and control the shape, size, and oxidizing or reducing characteristics of the flame desirable for particular applications.
More recently, burner systems for large industrial furnaces have been developed to burn pulverized coal fed from grinding mills using air as a transport medium, see Crites U.S. Pat. No. 1,541,903 of June 16, 1925, entitled "Means for Pulverizing, Feeding, and Burning Fuel". The carrier air is often referred to as "primary" air. The main combustion air is supplied to the burner as "secondary" air, and some attention has been given flame characteristics in the supply of such air. However, there is a lack of precise control of coal/air ratios in primary mixtures fed to the burner and of flow rates of secondary air. Achievable turndown ratio, i.e. ratio of maximum to minimum firing rate, is about three to one, and there is no control of flame shape for particular purposes. The lack of precise control in the aforementioned respects severely restricts selection and control of flame characteristics. Attempted use of commercially available equipment with greater turndown ratio results in unstable combustion or in flameout.
Since coal is usually stored in piles unprotected from the weather, it is often wet at the time of use. Pulverizing and burning systems are normally equipped with coal-drying equipment in advance of feed to the pulverizing mill or at least the carrier air is preheated.
Burners for pulverized solid fuels suspended in air have, in some instances, utilized a conical deflector rigidly mounted in a predetermined fixed position at the discharge end of and extending downstream from the firing conduit of the burner. Thus, in Smith et al. U.S. Pat. No. 4,221,174 of Sept. 9, 1980, entitled "Direct Ignition of a Fluctuating Fuel Stream", such a deflector is employed to diffuse a discharging stream of air-suspended pulverized coal with which is mixed oxygen or an inert gas at varying ratios said to provide optimum conditions for ignition of the discharged fuel mixture. Again, in Gunnerman U.S. Pat. No. 4,249,471 of Feb. 10, 1981 entitled "Method and Apparatus for Burning Pelletized Organic Fibrous Fuel", such a deflector is similarly employed to diffuse a stream of air-suspended pulverized sawdust, or similar organic fiber, with which is mixed a flammable gas for subsequent ignition and burning.
Pulverizers utilizing a staged impeller for impacting friable solid material to be ground and for throwing the impacted material outwardly against other stationary impacting members in an environment of turbulent air flow which promotes autogenous attrition of solid particles are well known in the pulverizing of materials such as lithopone, titanium oxide, cocoa, sulfur, talc and the like in instances where impalpable powders of five micron size or less are desired. For example, see Lykken et al. U.S. Pat. Nos. 2,392,331 and 2,497,088 and Jackering U.S. Pat. No. 3,071,330. However, pulverizers or ginding mills heretofore used in conjunction with burners for pulverized coal have been impact crushers adopted from the metallurgical industry, for example the hammer mill used in the system of the aforementioned Crites U.S. Pat. No. 1,541,903.